1. Field of the Invention
The present invention relates to an apparatus for testing the electrical integrity of an inductive coil, solenoid, or core and coil assembly. The invention specifically relates to such apparatuses which are compact and capable of providing visual displays corresponding to the test results.
2. Description of the Prior Art
It is well known that devices employing electrical coils are subject to numerous defects and failure during operation. Electrical coils are used to develop magnetic fields which can be used to perform functions such as counting the number of revolutions of a rotating member and actuating hydraulic cylinders. Additionally, electrical coils are often used to perform various functions in solid state devices such as television sets and display monitors.
In many of these applications, a failure of the coil or solenoid is often undetected, thereby leading to serious consequences. For example, if a magnetic sensor coil becomes short circuited or electrically open, it will not respond to the passage of the metal segment to induce a voltage pulse accordingly. The absence of this voltage pulse is often likely to go undetected. In a hydraulic cylinder, the solenoid may be short-circuited. This will result in the failure to actuate or deactuate a cylinder, thus leading to mechanical damage in the machine or to the product which it is producing. In such applications, the output voltage from the electrical system may vary from the nominal 12 volts depending on the electrical load on the system. The variable voltage being supplied to the solenoid makes it extremely difficult to design a test circuit for on-line testing of the solenoid's electrical integrity. Additional complications arise due to the fact that an on-line test must be such that it does not actuate the solenoid.
Defects may also occur during the manufacture of coils, the most common of which is that of a shorted turn or turns. Such a defect is relatively difficult to identify in a manner compatible with high volume production. One reason for this is that many apparatus lack the sensitivity to quickly isolate a single shorted turn within a coil containing several hundred turns. Another defect which may occur is that of corona discharge between high and low potential components.
With the advent of solid state television sets and cathode ray tube (crt) monitors, a need has arisen for a small, portable field instrument for testing inductive coils, including yokes and flyback transformers. In order to isolate or detect the cause of a problem in the sweep circuits of a television, it was common to substitute the inductive coil of a test instrument for the yoke of the television set or crt monitor, and subsequently measuring the amount of power delivered to the substitute inductive coil. If the problem were fixed, then it would be apparent that the yoke was defective. However, modern solid state televisions and crt monitors employ complex circuitry which make it difficult to provide a universal substitute inductive coil which will not overload the sweep circuits.
One method which has been reliably used to test yoke coils and transformer windings is a ringing test. Using this testing method, a narrow voltage pulse is applied to the inductive coil to cause it to ring, or resonate, thereby producing a dampened sine wave. Any defects in the inductive coil may be determined by analyzing the dampened sine wave on an oscilloscope. This test however is highly sophisticated and requires a skilled operator to properly adjust the oscilloscope. Furthermore, it is impractical for field operators to transport an oscilloscope to each test site.
The prior art makes many attempts at providing various apparatus and circuitry for testing inductive coils, solenoids, and the like. For example, U.S. Pat. No. 3,659,197 issued on Apr. 25, 1972 to Alley et al. discloses an apparatus for electrically testing an electrical coil. The apparatus includes a primary inductive coil and core assembly adapted to be magnetically coupled to the coil being tested. A pick-up coil is magnetically coupled to the core and coil assembly. A limited source of high voltage direct current is provided along with switching means in order to repetitively apply the current to the primary coil of the core and coil assembly. Detection means are electrically connected to the pick-up coil so that any defects in the coil being tested will be reflected back to the detection means via the pick-up coil.
U.S. Pat. No. 3,990,002 issued on Nov. 2, 1976 to Baum discloses a method and apparatus for testing television yokes and flyback windings. The apparatus includes a number of capacitors which are successively connected across a coil while a train of driving pulses is applied. The pulses cause the coil to ring after each driving pulse. The number of cycles which occur in the ringing signal between the two voltage levels thereof are counted to indicate whether or not the coil is defective.
U.S. Pat. No. 4,547,723 issued on Oct. 15, 1985 to McLellan discloses a device for detecting shorted turns in the windings of electromagnetic coils or transformers. The device utilizes the inductive kick voltage that is developed when the current through a coil is quickly interrupted. Rectified alternating current provides power while a high speed switch controls the current flow through the coil being tested. The device is also capable of measuring the voltage developed by the inductive kick.
U.S. Pat. No. 4,746,869 issued on May 24, 1988 to Schrag et al. discloses a circuit for detecting shorted turns in inductive coils and solenoids. The circuit includes a computer activated device for selectively imposing a direct current voltage on a coil. A detector is used to detect current flow through the coil induced by the voltage and for producing a current flow signal indicative of the magnitude of the current flow. A comparator compares the instantaneous signal with the reference signal and a computer determines whether the respective first and second comparison signals are produced during a first or second point in time, thus indicating the integrity of the coil.
U.S. Pat. No. 5,296,818 issued on Mar. 22, 1994 to Vrablec discloses an electrical tester for the control yoke of an oil-filled, electrically powered switch. The tester includes connectors which are adapted to be interfitted into electrical contact with the yoke. Indicators which are electrically connected to the connectors are used to indicate the flow of electrical current through the yoke. The apparatus may also be used to isolate the source of a fault in the transfer of control signals between the yoke and the electrical switch of the power distribution system.
None of the above inventions and patents, taken either singly or in combination, is seen to describe the instant invention as claimed.